Conventionally resin compositions containing epoxy resins, phenolic novolak resins, and cure promoters have been extensively used in the field of electric and electronic parts, especially as sealants for IC. In recent years, however, development of electronic materials have led to high level of storage and packing densities which in turn require particularly a sealant to have a higher resistance to heat and a lower water absorption. Above all, severe conditions such as immersion in a soldering bath as conducted in providing high density packing have posed an increasing need for cured substances to have a higher resistance to heat and a lower water absorption. However, conventional compositions such as cresol novolak epoxide resins commonly used as epoxy resins have insufficient heat resistance under the severe condition of immersion in a soldering bath. Similarly, polyepoxide compounds produced by epoxidation of polyphenols which are produced by condensation of phenols and aromatic aldehydes having a phenolic hydroxyl group have been proposed as those having a heat resistance as disclosed in Japanese Patent KOKAI (Laid-open) No. Sho 63-264622. However, they are less preferred in water absorption than cresol novolak epoxide resins.
Japanese Patent KOKOKU (Post-Exam.) No. Sho 62-20206 proposes polyglycidyl ethers derived from condensation of .alpha.-naphthol and form aldehyde as resins capable of producing cured substances excellent in heat resistance. The polyglycidyl ethers are excellent for an increased resistance to heat imparted by incorporation of naphthalene rings, but suffer from higher softening points, or higher melt viscosities caused by the incorporation of naphthalene rings resulting in an disadvantage of poor workability.
On the other hand, phenolic novolak resins commonly used as curing agent are still unsatisfactory in heat resistance. An attempt has been made to enhance the heat resistance by reducing the content of lower molecular weight components (such as binuclear phenol novolaks) still resulting in unsatisfactory resistance under severer conditions (for example, immersion in a soldering bath).